Combined Risk Index Report for Viti

Combined Risk Index Explanation

The Combined Risk Index is calculated by normalizing the values of the following columns:

• Rainfall in driest quarter: Lower value indicates higher risk
• Height above sea level: Lower value indicates higher risk
• Height above drainage network: Lower value indicates higher risk

Each column is normalized to a range between 0 and 1, where 0 represents the lowest risk and 1 represents the highest risk. The Combined Risk Index is the average of these normalized values.

Note that for the normalization:

• Height above sea level values above 10m are set to 10m to account for low risk from tidal surge/tsunami/sea level rise beyond this height.
• Height above drainage network values above 10 are set to 10 to account for low risk this high above drainage networks.
• Rainfall in driest quarter values above 800 are set to 800 to account for more ability to rely on rainwater

In the Pacific context, these variables are crucial due to their direct impact on communities' vulnerability to natural disasters, accessibility, and overall resilience:

• Rainfall in driest quarter: Essential for understanding drought risks and scope for people to rely on rainwater harvesting.
• Height above sea level: Elevation is critical in assessing flood and storm surge risks. Lower elevations are more vulnerable.
• Height above drainage network: Directly measures the susceptibility to flooding events, a significant hazard in the Pacific.

High Risk Sites Classification

High risk sites are classified as follows:

• Rainfall in driest quarter: Sites with values below 400mm.
• Height above sea level: Sites with values below 5m above sea level.
• Height above drainage network: Sites with values below 5m above nearest drainage network.

The percentage of high-risk sites is calculated as the number of high-risk sites divided by the total number of sites.

Methodology Documentation

Rainfall in driest quarter: Data sourced from WORLDCLIM, calculated using precipitation of the driest quarter.

Height above sea level: Data sourced from MERIT Hydro DEM, calculated using elevation values. The height above sea level (HASL) provides information about the elevation of a location relative to mean sea level. It's a critical factor for assessing flood and storm surge risks. Lower HASL indicates higher vulnerability. HASL's advantage is its direct relation to flood risks from sea level rise and storm surges. However, it does not account for local topography and drainage patterns which can affect flood risks.

Height above drainage network: Data sourced from MERIT Hydro DEM, calculated using height above nearest drainage (HAND). The height above drainage network (HAND) provides a measure of the elevation of a location relative to the nearest drainage point. It is essential for understanding flood risks from rainfall events. Lower HAND values indicate higher flood risk. HAND's advantage is its ability to account for local topography and drainage patterns. However, it does not directly relate to risks from sea level rise or storm surges.